Induction heater having a conductor with a radial heating element

ABSTRACT

An induction heater is provided which has an electrical conductor (40) extending along an axis B. At least one solid heating element (43) for contacting and transferring heat to a material to be heated extends radially from the axis B and is formed of electrically conductive and preferably ferromagnetic material. An alternating electric current is supplied to the electrical conductor (40) to induce an electric current in a closed loop defined by a surface layer of the or each heating element (43) to generate heat therein. Induced eddy currents are confined to a surface layer due to the skin effect. The radial and axial thickness of the heating elements (43) is greater than twice the skin depth of the electrically conductive material at the frequency of the alternating electric current. Preferably the heating element (43) is in the form of a screw arranged on an electrically conductive heating member (42).

This invention relates to an induction heater wherein material is heatedby contact with an inductively heated heating element.

Bulk or continuous flow heaters are employed as driers or calciners.Typically, a heating member contacts material to be heated so that heatis efficiently transferred from the heating element to the material tobe heated. Some degree of mixing action to improve contact can also beincorporated with the heating action of the heating element. This canfor example be provided by the heating element with fin members. As aresult, contact between the heating element and the material to beheated is enhanced. However, it is preferable to provide substantialuniform heating of the material to be heated. Consequently means arerequired to supply heat uniformly to the heating member and the finmembers thereof.

The supply of heat to the heating element is particularly problematicalwhere fin members are included. Known driers incorporate heating bymeans of gas jets or hot fluid. Consequently, in order to heat such finmembers complicated supply tubing must be engineered into the finmember. It is also known to adapt the fin member to enhance the mixingresulting from movement of the heating element. This only serves tofurther complicate the design of the fin member. Particular problems areencountered with rotary drum driers and calciners since it is necessaryto incoporate rotary couplings for supply of the hot fluid or gases.Consequently, there are a number of drawbacks for heating the heatingelement of known bulk continuous flow heaters.

GB 2163930 discloses an induction heater having an alternating currentcarrying conductor extending along an axis. A core means substantiallyencircles the axis to guide magnetic flux resulting from an alternatingcurrent in the conductor. The heating element is an electrically heatingclosed loop which encircles the magnetic flux in the core means and sois heated by electrical current induced therein.

Two of the embodiments disclosed in GB 2163930 are shown in FIGS. 1 and2. In the embodiment of FIG. 1, the conductor 1 forms an axis aboutwhich is provided a ferromagnetic core 4. The core 4 is enclosed withina metal skin formed from concentrically aligned inner cylinder 5 andouter cylinder 7 and end plates 6 and 9. In this way, the skin forms aclosed electrically conducting loop about the core 4. Alternatingcurrent set up in the conductor 1 by a toroidally wound transformer 8set up an alternating magnetic flux which is guided by the core 4. Inturn, the alternating flux in core 4 induces currents to flow around theabove-mentioned electrically conducting closed loop. The material to beheated is placed within the inner cylinder 5 and is heated by the energyproduced in the cylinder by the induced currents. The structurecomprising the cylinders 5 and 7 and core 4 can be rotated in thedirection of the arrow A. In this way the material to be heated is movedinto and out of contact with the cylinder 5 to allow uniform transfer ofthe heat from the cylinder to the material to be heated.

FIG. 2 shows a continuous flow induction heater. A motor 21 rotates ascrew structure 22 in the direction of the arrow A. The screw structure22 comprises an outer wall 23 which has a spiral slot cut in it toreceive screw flight 24. The structure 22 also has an inner wall 25. Atoroidal ferromagnetic core 26 is sandwiched between the inner and outerwalls 23, 25. An electrically conducting conductor 1, corresponding tothat shown in FIG. 1, runs along the axis of the structure 22. As in theembodiment of FIG. 1, a magnetic flux is induced in the core 26 by analternating current in the conductor 1; this magnetic flux, in turn,induces electrical currents to flow in the walls 23, 25 and the screwflight 24 of the structure 22. The structure 22 is located within a can28 having an inlet 29 and outlet 30 as shown. Consequently materialentering at 29 contacts the structure 22 and is urged towards outlet 30by the screw flight 24 as the structure 22 is rotated. The material isheated while in contact with the structure 22.

This structure is however quite complicated requiring a toroidalferromagnetic core 26 to be sandwiched between inner and outer walls 23and 25. It is an object of the present invention to provide a simplifiedinduction heater.

The present invention provides an induction heater comprising anelectrical conductor extending along an axis; at least one solid heatingelement for contacting and transferring heat to a material to be heated,the or each heating element extending radially from said axis, and beingformed of electrically conductive material; and means to apply analternating electric current to said electrical conductor to induce anelectric current in a closed loop defined by a surface layer of the oreach heating element to generate heat therein, the radial and axialthickness of the or each heating element being greater than twice theskin depth of the electrically conductive material at the frequency ofthe applied alternating electric current.

The skin depth of a material is the depth of material from the surfaceat which the magnetic field and current density fall to l/e of theirsurface values.

The arrangement of the present invention requires no toroidalferromagnetic core. The or each heating element can be heated when noelectrically conductive loop is provided. This is due to eddy currentswhich are induced in a surface layer defined by the skin depth of thematerial of which the heating element is constructed; the skin depthbeing a function of the magnetic and electric properties of theelectrically conductive material from which the heating element isformed as well as the frequency of the applied field produced by thealternating current. The current will flow in a surface layer of theradially extending heating element. Thus the surface of the heatingelement is heated by this electrical current and a large heating surfacearea can be presented to the material to be heated.

Preferably the or each heating element is formed of an electricallyconductive and ferromagnetic material, since the skin depth will besmaller and greater heating will occur.

In one embodiment of the present invention the or each heating elementis adapted to move relative to said axis and to material brought intocontact with the or each heating element.

Preferably the radial thickness of the or each heating element is verymuch greater than said axial thickness. This ensures that the radialheating effect is greater than the heating effect along the axis.

In one embodiment of the invention there is provided a heating memberextending substantially parallel to the axis and the or each heatingelement extends radially and preferably substantially perpendicular fromsaid heating member.

The heating member can conveniently take the form of a cylinderencircling the electrical conductor and adapted to rotate about theaxis. The heating member can either be formed to an insulating material,an electrical conductive material, or an electrical conductive andferromagnetic material. If the heating member is electrically conductivethen preferably the radial thickness of the heating member is greaterthan twice the skin depth of the electrically conductive material at thefrequency of the applied alternating electric current.

In one embodiment of the present invention the or each heating elementcomprises a fin member and the or each fin member is angled such thatwhen the heating member is rotated the material to be heated is urgedalong the axis of rotation by the or each fin member.

Preferably at least one heating element forms a screw. The screw can beattached to the heating member and when the heating member is rotated,it can urge the material to be heated along the axis of rotation.

In another embodiment of the present invention the or each heatingelement extend from an outer surface of the heating member to heat amaterial provided about the outer surface. This arrangement keeps thematerial to be heated away from the electrical conductor and this helpsto keep the electrical conductor cool. Such cooling can be improved bythe passage of a coolant. Preferably the electrical conductor comprisesa tube and the coolant is passed along the axis through the tube.

In an alternative embodiment of the present invention the or eachheating element extends from an inner surface of the heating member.

Examples of the present invention will now be described with referenceto the drawings, in which:

FIGS. 1 and 2 illustrate prior art induction heaters as describedhereinabove;

FIG. 3 illustrates a cross-section of one embodiment of the presentinvention;

FIG. 4 functionally illustrates a cross-section through the wall of theheating member of FIG. 3; and

FIG. 5 illustrates a cross section through an induction heater accordingto a second embodiment of the present invention.

Referring now to FIG. 3, this is a schematic longitudinal sectionthrough a rotatable induction heater according to one embodiment of thepresent invention. As shown in FIG. 3 the induction heater includes anelectrical conductor 40 capable of conducting alternating electriccurrents and which extend along the axis of rotation B of the inductionheater. The electrical conductor 40 which is typically made of coppermay be laminated to reduce its AC resistance is connected to a source ofalternating current 41. The electrical conductor 40 extends through asolid elongate heating member 42 in the form of a cylinder with an outersurface remote from the electrical conductor 40 having a heating elementforming a screw 43. The heating member 42 with screw 43 is rotatableabout the electrical conductor 40 and axis B. The heating member 42 andscrew 43 are housed within a housing 44 which has an inlet 45 and outlet46 for the material to be heated. The screw 43 is formed preferably of aferromagnetic material, to provide greater heating. The heating member42 may also be formed of ferromagnetic material, although if it isformed of non-ferrous material the screw 43 desirably provides greaterheating than the heating member 42.

When an electric current is passed along the electric conductor 40 andthe heating member 42 is rotated, the material to be heated is drawnthrough the inlet 45 and urged by the screw 43 in the direction of theaxis of rotation B to the outlet 46. During passage of the material tobe heated electrical currents induced in the heating member 42 and screw43 generate the heat to heat the material. The action of the screw 43therefore serves to mix the material to be heated during its passage toensure even heating.

FIG. 4 illustrates the currents induced in the wall of the heatingmember 42 and in the screw 43. When the alternating current flows downthe conductor 40 a circumferential magnetic field is produced whichinduces eddy currents in a surface layer 50. The induced current on aface proximate to the electrical conductor 40 is opposite in direction.Induced currents are confined to a surface layer due to the skin effect.Thus currents travel parallel to the axis in a closed loop formed by theinner and outer surface layers of the heating member 42. The arrows 51indicate the direction of the skin currents. At the position of aheating element comprising part of the screw 43, due to the skin effectthe induced currents remain in a surface layer and are thereforeconducted around the surface of the screw 43 rather than across the baseof the screw 43. There is thus an even current over the surface of theheating member 42 with screw 43. Thus the material to be heated incontact with the screw 43 is heated by induced eddy currents. Theheating provided over the surface of the heating member 42 with screw 43provides a large heating surface area.

FIG. 4 only illustrates the instantaneous eddy currents which willalternate with the polarity of the alternating current in the electricalconductor 40.

For the currents in FIG. 4 to flow there must be no short circuit eitherradially across the heating member 42 or axially across the screw 43.For this condition to be met the thickness of heating member 42 and thescrew 43 must be greater than twice the skin depth of the magnetic fieldat the frequency of the alternating current. The skin depth will vary asa function of the electrical and magnetic properties of the material aswell as the frequency of the applied magnetic field produced by thealternating current.

In the arrangement shown in FIG. 3, the screw 43 is arranged on a faceremote from the electrical conductor 40. The material to be heated iskept away from the electrical conductor 40. The electrical conductor 40is thus kept cooler since it is not in contact with the material to beheated. Cooling can be further improved by passage of a coolant such ascooling water through the tubular construction of the electricalconductor 40.

Although the example described hereinabove utilises a screw, heatingelements protruding from the heating member 42 could comprise any shapedelement that increases the surface area of the heating member 42 andhave a radial thickness very much greater than the axial thickness.Ideally these should protrude substantially vertically from the heatingmember to gain maximum heating effect. For instance, fin members couldbe used to also propel the material to be heated from the inlet 45 tothe outlet 46. Where the material to be heated need not be propelled bythe heating member 42, the heating element need not be shaped to urgethe material.

To increase the heating of the heating member 42 at a surface remotefrom the electrical conductor 40, the heating member 42 can beconstructed such that the surface remote from the electrical conductor40 and/or the screw 43 can be constructed from a material of differentelectrical resistance to the surface proximate to the electricalconductor 40. This arrangement will provide for an differential heatingon the outer face in contact with the material to be heated. The skindepth for this material is likely to be different and its thickness mustbe larger than twice the skin depth in order to ensure that a currentcan flow in the surface layers.

Although the example described hereinabove utilises an electricallyconductive heating member 42, any material can be used for the heatingmember 42. If an insulating material is used, eddy currents induced inthe heating elements 43 flow radially inwardly and outwardly, with acurrent flowing across the base of the heating element 43 where it joinsthe heating member 42. In the example described above, for anelectrically conductive heating member 42, the eddy currents induced inthe heating elements 43 and the heating member 42 can be considered tobe equal and opposite at the base of the heating element 43, where itjoins the heating member 42.

The induction heater will also operate without a heating member 42,since the eddy currents induced in the heating elements are of primaryconsideration for the heating effect. However, practically some supportfor the heating elements must be provided.

In an alternative arrangement to the embodiment described hereinabove,as shown in FIG. 5, the heating element 143 is provided extending fromthe inner surface of the heating member 142. In such an arrangement thematerial to be heated is passed through the inside of the heating member142. No housing 44 need be provided. The heating element 143 in thisarrangement is conveniently in the form of a helical screw. In a likeconfiguration to the embodiment shown in FIG. 3 an alternating currentsource 141 supplies a current to an electrical conductor 140 throughwhich cooling water passes in use. In this embodiment the heating member142 with internal screw 143 rotates about the axis B thus heatingmaterial passing therethrough. The arrows in FIG. 5 refer to thedirection of flow of the material to be heated.

I claim:
 1. An induction heater for heating a material comprising anelectrical conductor extending along an axis; at least one solid heatingelement for contacting and transferring heat to said material to beheated, said at least one heating element extending radially from saidaxis, and being formed of electrically conductive material; and means toapply an alternating electric current to said electrical conductor at afrequency to induce an alternating electric current in said at least oneheating element to generate heat therein, said means to apply thealternating electric current being arranged to generate the alternatingelectric current at a frequency wherein the induced alternating electriccurrent flows in a closed circuit comprising a surface layer of said atleast one heating element, said surface layer having a thickness of theskin depth of the electrically conductive material at said frequency;the radial and axial thickness of said at least one heating elementbeing greater than twice said skin depth, and said radial thickness ofsaid at least one heating element being substantially greater than saidaxial thickness.
 2. An induction heater as claimed in claim 1, whereinthe heating element is formed of electrically conductive andferromagnetic material.
 3. An induction heater as claimed in claim 1,wherein the heating element is adapted to move relative to said axis andto said material to be heated which is brought into contact with theheating element.
 4. An induction heater as claimed in claim 1 includinga heating member extending substantially parallel to said axis, whereinthe heating element extends radially from said heating member.
 5. Aninduction heater as claimed in claim 4, wherein the heating elementextends substantially vertically from said heating member.
 6. Aninduction heater as claimed in claim 4, wherein said heating memberencircles said electrical conductor to form a cylinder and is adapted torotate about said axis.
 7. An induction heater as claimed in claim 6wherein each fin member is angled such that when the heating member isrotated the material to be heated is urged along the axis of rotation byeach fin member.
 8. An induction heater as claimed in claim 6, whereinthe heating element extends from an inner surface of said heatingmember.
 9. An induction heater as claimed in claim 4, wherein saidheating member is electrically conductive.
 10. An induction heater asclaimed in claim 9, wherein the radial thickness of the heating memberis greater than twice the skin depth of the electrically conductivematerial at the frequency of the applied alternating electric current.11. An induction heater as claimed in claim 4, wherein said heatingmember is electrically conductive and ferromagnetic.
 12. An inductionheater as claimed in claim 4, wherein the heating element extends froman outer surface of said heating member to heat material at an outerside of said heating member.
 13. An induction heater as claimed in claim12, wherein said electrical conductor is cooled by the passage of acoolant.
 14. An induction heater as claimed in claim 13, wherein saidelectrical conductor comprises a tube and said coolant is passed alongsaid axis through said tube.
 15. An induction heater as claimed in claim1 wherein the heating element comprises a fin member having a radialface which is substantially larger than the axial thickness of said finmember.
 16. An induction heater as claimed in claim 1 wherein said atleast one said heating element forms a screw.